It is common to use a single antenna array to provide a radiation pattern, or beam, which is steerable. For example, steerable beams are often produced by a planar or panel array of antenna elements each excited by a signal having a predetermined phase differential so as to produce a composite radiation pattern having a predefined shape and direction. In order to steer this composite beam, the phase differential between the antenna elements is adjusted to affect the composite radiation pattern.
A multiple beam antenna array may be created, utilizing a planar or panel array described above, for example, through the use of predetermined sets of phase differentials, where each set of phase differential defines a beam of the multiple beam antenna. For example, an array adapted to provide multiple selectable antenna beams, each of which is steered a different predetermined amount from the broadside, may be provided using a panel array and matrix type beam forming networks, such as a Butler or hybrid matrix.
When a planar array is excited uniformly (uniform aperture distribution) to produce a broadsided beam projection, the composite aperture distribution resembles a rectangular shape. When this shape is Fourier transformed in space, the resultant pattern is laden with high level side lobes relative to the main lobe. Moreover, as the beam steering increases, i.e., the beam is directed further away from the broadside, these side lobes grow to higher levels. For example, a linear array with its beam-peak at .THETA..sub.o can also have other peak values subject to the choice of element spacing "d". This ambiguity is apparent, since the summation also has a peak whenever the exponent is some multiple of 2.pi.. At frequency "f" and wavelength lambda, this condition is 2.pi.(d/.lambda.)(sin.THETA..sub.scan -sin.THETA..sub.O)=2.pi.p for all integers p. Such peaks are called grating lobes and are shown from the above equation to occur at angles .THETA.p such that sin.THETA..sub.p =sin.THETA..sub.O =2.pi.p. Accordingly, when the radiation pattern is steered too far relative to the element spacing a grating lobe will appear which can have a peak in its pattern nearly equal to the main lobe of the radiation pattern. The point at which this occurs is generally considered the maximum useful steering angle of the array.
Even when steering of the main beam is restricted to angles such that the grating lobe presents a peak appreciably less than that of the main lobe, the presence of the grating lobe acts to degrade the performance of the antenna system by making it responsive to signals in an undesired direction, potentially interfering with the desired signal. Specifically, as the main beam is steered off of the broadside of the array, the grating lobe will often be directed at an angle within the range of angles the antenna array is operable within. Accordingly, the presence of a stray communication beam having a substantial peak associated therewith and present within the area of operation of the antenna array will very often be a source of interference. Moreover, as the grating lobe is substantially coaxial with the axis of radiation of the antenna panel, it is generally not possible to avoid this interference with solutions such as tilting the array to point the grating lobe in a harmless direction.
Additionally, broadside excitation of a planar array yields maximum aperture projection. Accordingly, when such an antenna is made to come off the normal axis, i.e., steered away from the broadside position which is normal to the ground surface and centered to the surface itself, the projected aperture area decreases causing a scan loss. This scan loss further aggravates the problems associated with the grating lobes because not only is the aperture area of the steered beam decreased due to the effects of scan loss, but the unwanted grating lobes are simultaneously increased due to the effects of beam steering.
Accordingly, a need exists in the art for a system and method of providing antenna beams having a desired beam widths and azimuthal orientations without suffering from the presence of grating lobes when steered a desired amount off of the broadside.
Moreover, as multiple beam antenna arrays are useful in providing wireless communication networks, such as cellular and/or personal communication services (PCS) networks (referred to hereinafter collectively as cellular networks), which are often simultaneously provided in a same service area, a need exists in the art for the systems and methods adapted to provide desired antenna beams substantially free of grating lobes to also be adapted for dual mode service.